The present invention relates to a method and apparatus for measuring the contrast sensitivity of the human eye.
In recent years, it has been recognized that contrast is an important parameter for assessing vision. Although visual acuity tests can be used to detect changes in the spatial resolving capability of the human eye, used alone it is typically inadequate for measuring the eye""s performance when subjected to glare or reduced light environments and for measuring vision loss not caused by refractive error. Often, the human eye will lose its ability to perceive patterns, i.e., contrast, even though visual acuity is still normal. Indeed, the first indication of certain eye pathologies is a loss of contrast sensitivity.
Contrast sensitivity determines the lowest contrast level which can be detected by a subject for a given size target, defined in terms of spatial frequency. Normally, a wall-mounted chart printed with circular patches of sine wave gratings, which decrease in contrast from left to right, and increase in spatial frequency from top to bottom, is used to measure contrast sensitivity. The gratings are either vertical, or tilted at 10 degrees to the left or right of vertical. During an examination, the subject is asked to identify the orientation of the grating patches, moving from left to right for each row. For each row or spatial frequency, the contrast level of the last grating patch""s orientation correctly identified is the contrast threshold. The reciprocal of this contrast threshold is commonly known as the contrast sensitivity.
Efforts to automate contrast sensitivity measurements include the use of computer controlled monitor-based systems. In these latter systems, vertical sinusoidal gratings are displayed on a high-resolution monitor controlled by a computer, again for varying contrast levels. This is done by incrementally increasing the spatial frequency in predetermined steps. More specifically, while incrementing spatial frequency, the contrast level is varied according to a contrast control algorithm for determining the minimum amount of contrast needed for the subject to see the grating. As noted, this minimum contrast level is called the contrast threshold. Typically, the contrast control algorithm either increases or decreases the contrast level according to whether the previous grating was seen or not seen, and continues until the contrast level oscillates about the contrast threshold. Typically, when the subject correctly responds three times, then the contrast level is decreased by one step. An incorrect response leads, however, to a one step increase in the level.
Another approach involves progressively increasing the contrast level until the subject reports seeing the grating, and then progressively decreasing the contrast until the subject reports not seeing the grating. The contrast threshold is taken as the average of the contrast levels of when the subject reports seeing and not seeing the grating.
Although such contrast sensitivity measurement systems as well as other psychophysical methods work reasonably well, it is somewhat time-consuming for them to measure the contrast threshold with a high degree of accuracy. Accordingly, it would be desirable to have a method and system for measuring the contrast sensitivity which is more efficient and accurate.
A novel contrast sensitivity test utilizing an adaptive contrast threshold algorithm is proposed. The contrast sensitivity test is embodied as a two-alternative forced choice examination using grating stimuli of randomly interleaved spatial frequencies. The stimulus""s orientation is either tilted to the left or right of vertical. During an examination, a succession of grating stimuli is presented to the subject, and the subject asked to indicate the orientation of each stimulus. The contrast level for the next stimulus is varied depending on the subject""s response to the previous one using an adaptive weighted up-down algorithm. For each correct response, the contrast level is reduced, whereas for each incorrect response the contrast level is increased. Importantly, the amount by which the contrast level is lowered is different from the amount by which it is raised, and as the examination progresses, the amount changes. A maximum likelihood method is then employed for finding the best estimation of the contrast threshold from the subject""s responses to known contrast levels.
In one embodiment, the grating stimulus consists of dark and light bars having a luminance which varies in a sinusoidal manner. Alternatively, the luminance can vary in a square-wave manner. In another embodiment, the grating stimulus consists of color bars either tilted to the left or right of vertical. Either the luminance or saturation can be varied sinusoidally from a maximum to a minimum. Likewise, a square wave color grating may also be used. Characteristic changes in the subject""s contrast sensitivity for the different spatial frequencies may be used to detect certain eye diseases and disorders.